A conventional combined cycle power plant generally includes a gas turbine, a heat recovery steam generator (HRSG), and a steam turbine. Exhaust from the gas turbine may be used to generate steam which in turn expands in the steam turbine to produce work. For example, expansion of the steam in the steam turbine may rotate a rotor connected to a generator to produce electricity.
The steam turbine and rotor may be subjected to substantial thermal transients, particularly during heat-up of the steam turbine. The thermal transients in turn may create substantial thermal stresses along the rotor which, if not carefully controlled, may reduce the low cycle fatigue and/or useful life of the rotor. As a result, heat-up limits may be applied to the steam turbine to ensure that the maximum designed heat-up rates of the steam turbine and/or rotor are not exceeded. Since the exhaust from the gas turbine is typically used to heat the steam turbine, the heat-up limits may in turn be applied to the gas turbine. In many cases, the heat-up limits applied to the gas turbine are not associated with actual measured parameters of the gas turbine and are instead “surrogate” limits which, if met, will ensure that the actual limits are also not exceeded. The surrogate limits may include, for example, limits or holds on the power output of the gas turbine and/or on the fuel flow to the gas turbine.
Although effective at limiting the heat-up rate of the steam turbine and rotor, in some instances the surrogate limits applied to the gas turbine may be slightly inaccurate and/or unnecessarily restrictive as a result of changes in local operating conditions. For example, changes in the ambient temperature or humidity may produce corresponding changes in the gas turbine exhaust temperature and/or exhaust flow rate for a given power output and/or fuel flow. As a result, the surrogate limits on the power output of the gas turbine and/or the fuel flow to the gas turbine may not produce the desired exhaust temperature and/or exhaust flow rate to the heat recovery steam generator to achieve the optimum heat-up rate in the steam turbine. Therefore, an improved system and method for warming up a steam turbine and rotor would be useful.